The openings on the aircraft skin are normally identified by the term manhole openings, whereas the covers of these openings are called manhole covers or doors for manhole openings. Thus manhole openings and manhole covers in aircraft are commonplace and are necessary both for assembly and the subsequent inspections and maintenance tasks throughout the life of the aircraft.
In the specific case of fuel tank inlets on aeronautic structures, aircraft wings and stabilisers, this access is usually carried out through manhole openings which, furthermore, must comply with several requirements:                The size of the manhole opening must be in accordance with the accessibility requirements        The door or manhole cover must be sealed so as to prevent fuel loss        They must be in compliance with impact requirements        Assembly and disassembly of doors or manhole covers must be as quick and easy as possible        The doors or manhole covers must be interchangeable        
One of the known solutions for aircraft manhole openings employs an opening having adequate dimensions, in which two separate manhole covers or doors are installed, one manhole cover located inside the fuel tank and a second manhole cover on the exterior aerodynamic surface of the aircraft, both manhole covers screwed together in such a way that they hold the panel of the skin of the aircraft between them. To maintain the outer aerodynamic surface, a cavity is machined on the outer face of the skin which will be used for the installation of the outer manhole cover. At the same time, a sealing ring is employed to seal the inner manhole cover to the inner surface of the skin, after which both manhole covers are screwed together to close the manhole opening.
Thus current practice is to employ some sealing rings to seal the manhole covers against the skin of the aircraft structure that surrounds the manhole opening at issue. Currently the majority of aeronautic structures are made of composite materials, which must maintain specific behavioral specifications in the event of a lightning strike. For this reason, said sealing rings are not structural parts and are usually made of fiberglass due to the behavioral requirements they must have to be of service against lightning strikes.
As has been mentioned, these sealing rings must be able to seal the inner manhole cover with the inner surface of the skin. The surface of the sealing ring in contact with the outer skin surface of the aircraft structure has a shape in a double curve, such that it moulds itself to said surface. The opposing surface of this sealing ring is flat.
In this way, given that the positions of the manhole openings require a different shape from the inner surface of the skin, a sealing ring is required that has a different shape for each position of the manhole openings of the aircraft, because the shape of the inner surface of each ring varies and is moulded to the shape of the inner surface of the skin. Therefore, there is a very high number of differing shape designs for these sealing rings, which carries over to high production and engineering costs. Likewise, errors may occur during production and assembly of these rings, given the large diversity of parts that must be managed. Furthermore, there is no possibility of interchanging these parts, which further elevates the costs of the process.
The present invention is directed towards providing a solution to the problems described above.